The chairman of the House Intelligence Committee, Rep. Adam Schiff, D-Calif., says his committee is moving "expeditiously" on hearings and subpoenas. That committee, as well as the House Oversight and Reform Committee and the House Foreign Affairs Committee, have scheduled depositions starting this week for State Department officials linked to Trump's dealings with Ukraine.

Impeachment Probe Persists Despite Recess

Members of the House Intelligence Committee have been told to be prepared to return to Washington during the break. California Rep. Jackie Speier said she has already canceled some of her previous commitments.

"We're expected to be here," Speier said.

House Speaker Nancy Pelosi has told the Democrats they need to "strike while the iron is hot" on impeachment, sending the committees into overdrive. Connecticut Rep. Jim Himes, a Democrat, said a plan is "being formed very rapidly."

"What I know for sure is that momentum will not slow," Himes said.

Rep. Raja Krishnamoorthi, D-Ill., said they will have to "work harder" and "sleep less."

Long Witness List, Quick Timeline

Schiff's committee has been negotiating to interview the whistleblower who began the firestorm by reporting to the inspector general for the intelligence community that Trump had urged the investigations on a July phone call with Zelenskiy.

Schiff told ABC's "This Week" that his panel had reached agreement to hear from the whistleblower, who would testify "very soon." Schiff said the exact date would depend in part on how quickly acting Director of National Intelligence Joseph Maguire completes the security clearance process for the whistleblower's lawyers. "We'll keep obviously riding shotgun to make sure the acting director doesn't delay in that clearance process," Schiff said.

The inspector general who handled that complaint, Michael Atkinson, is slated to testify to the Intelligence Committee in private on Friday, according to a person familiar with the committee who was spoke on condition of anonymity to discuss internal deliberations.

Lawmakers on the committee say they also want to speak to White House aides who were present for the call and to Rudy Giuliani, the president's personal lawyer, who urged the investigations. Giuliani told ABC on Sunday that he "wouldn't cooperate" with Schiff, but if Trump "decides that he wants me to testify, of course I'll testify." Schiff says he hasn't decided whether he wants to hear from Giuliani.

Democrats say they hope to finish the investigation in a matter of weeks — perhaps even before Thanksgiving.

Drafting Articles of Impeachment

Once the committees have finished their own investigations, the committees will submit their findings to the House Judiciary Committee, which oversees the impeachment process.

Rep. Pramila Jayapal, D-Wash., who serves on the Judiciary Committee, said the Intelligence Committee will be the "star of the show" as it investigates Trump's activities related to Ukraine. Articles of impeachment would be drafted by the Judiciary Committee and, if adopted, sent to the House floor.

The Judiciary Committee chairman, Rep. Jerrold Nadler, D-N.Y., has said he wants resolution on impeachment by the end of the year. Jayapal said that deadline "absolutely" stands, and that the plan is to be done before January, or "perhaps sooner."

Republican Resistance

Republicans have focused their ire about impeachment on the Democrats, criticizing the probes as a rerun of a two-year investigation into Russian election interference in the 2016 election.

California Rep. Devin Nunes, the top Republican on the Intelligence Committee, said Democrats "don't want answers, they want a public spectacle."

"They have been trying to reverse the results of the 2016 election since President Trump took office," said House Minority Leader Kevin McCarthy, R-Calif.

Senate Has 'No Choice' But to Hold Trial

If the House votes to approve charges against Trump, the Republican-led Senate would then hold a trial.

Some Senate Republicans have expressed concerns about Trump's interactions with Ukraine, but there are few signs that there would be enough discontent to convict the president, who still has strong support in the GOP ranks. If Trump were impeached, it would take a two-thirds vote in the Senate to convict him and remove him from office. A memorandum from Senate Republicans circulated over the weekend acknowledged it would be hard for McConnell to block an impeachment trial, but he could deflect any House-approved impeachment articles to a committee.

Senate Majority Leader Mitch McConnell said he would have "no choice" but to take up any House-approved impeachment articles against Trump, telling CNBC in an interview Monday that the Senate rules require him to do so.

The chairman of the Senate Intelligence Committee, Sen. Richard Burr, R-N.C., has said his committee will investigate the Ukraine matter but "don't expect us to move at light speed — that will probably happen in the House."

How Many Presidents Have Been Impeached?

Trump would join a rare group if the House moves forward toward impeachment. Only two presidents have been impeached: Andrew Johnson in 1868 and Bill Clinton in 1998. Both won acquittal in the Senate.

President Richard Nixon, who faced impeachment proceedings, resigned from office in 1974.